Sinterability and Piezoelectric Properties of (K,Na)NbO₃ Ceramics with Novel Sintering Aid

Dense (K,Na)NbO₃ (KNN)-based ceramics were developed by optimizing the sintering conditions with newly developed K₄CuNb₈O₂₃ (KCN) as a sintering aid. The sinterability and electrical properties of KNN ceramics were investigated as a function of KCN concentration. The density of KNN ceramics increased monotonically with increasing the KCN contents, and reached the highest value of 4.40 g/cm³ with the addition of 0.5 mol% KCN. This is due to the formation of liquid phase, which significantly promotes the densification of KNN ceramics. The Curie temperature (T_c) of pure KNN ceramics was 420°C and decreased with increasing the KCN content. With the addition of 0.5 mol% KCN, planar mode electromechanical coupling factor (k_p) and mechanical quality factor (Q_m) of KNN ceramics also reached the highest values of 0.39 and 1200, respectively. The KNN ceramics prepared with 0.5 mol% KCN exhibited a large field-induced strain of 0.09% at 40 kV/cm and the piezoelectric constant d₃₃ of 180 pm/V.